下面是在.h 头文件里对一个32位寄存器的定义，有些地方不明白, 请教一下，先谢谢啦！
1）为什么这里typedef union有4个 struct? 是不是针对不同情况？如果我使用第一个struct，就不能用其他的么？
2）如果我对某一位赋值，我可以使用, 比如PMCONbits.CS2P=1；（因为定义了unsigned CS2P:1;）
但是有些位没有定义，比如unsigned:1;是不是说明该位不能被赋值？如果能，怎么赋值？
3） 能不能用PMCON对整个寄存器赋值，比如PMCON=0xFFFFFF00
4) PMCONCLR是不是所有为置0， PMCONSET所有为置1？
#define PMCON PMCON
extern volatile unsigned int PMCON __attribute__((section("sfrs")));
typedef union {
struct {
unsigned RDSP:1;
unsigned WRSP:1;
unsigned :1;
unsigned CS1P:1;
unsigned CS2P:1;
unsigned ALP:1;
unsigned CSF:2;
unsigned PTRDEN:1;
unsigned PTWREN:1;
unsigned PMPTTL:1;
unsigned ADRMUX:2;
unsigned SIDL:1;
unsigned :1;
unsigned ON:1;
unsigned :1;
unsigned DUALBUF:1;
unsigned :5;
unsigned RDSTART:1;
};
struct {
unsigned :6;
unsigned CSF0:1;
unsigned CSF1:1;
unsigned :3;
unsigned ADRMUX0:1;
unsigned ADRMUX1:1;
};
struct {
unsigned :13;
unsigned PSIDL:1;
unsigned :1;
unsigned PMPEN:1;
};
struct {
unsigned w:32;
};
} __PMCONbits_t;
extern volatile __PMCONbits_t PMCONbits __asm__ ("PMCON") __attribute__((section("sfrs")));
extern volatile unsigned int PMCONCLR __attribute__((section("sfrs")));
extern volatile unsigned int PMCONSET __attribute__((section("sfrs")));
extern volatile unsigned int PMCONINV __attribute__((section("sfrs")));
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